Photodynamics of MALDI matrix molecules studied by velocity-map imaging

Matrix assisted laser desorption ionization (MALDI) is widely used within the mass spectrometry community to generate gas-phase samples of intact biomolecules and other non-volatile molecules. The sample of interest is embedded in or coated with a suitable 'matrix compound', often an organic acid, with a strong optical absorption in the ultraviolet (UV). On irradiation with UV light, the matrix absorbs the light preferentially, thereby protecting the delicate biomolecule from radiation damage. Absorption of a photon triggers the transfer of energy and charge from the matrix compound to the biomolecule, which is then ejected from the surface, yielding gas-phase biomolecules suitable for analysis by mass spectrometry. While the basic principle of the MALDI process is widely appreciated, the detailed molecular mechanism is not well understood. We plan to adopt a 'bottom up' approach to understanding the MALDI mechanism. Initially, we will study the gas-phase photochemistry of isolated MALDI matrix molecules in order to understand their decomposition pathways following absorption of UV light. We will employ multimass velocity-map imaging (VMI) to measure time-of-flight mass spectra and product scattering distributions for each photofragment, allowing the fragments to be identified and quantified and the fragmentation dynamics to be investigated in detail. We will investigate the wavelength dependence of the process, which will provide information on the character of the electronic states involved, and will also study the dependence on molecular structure by performing measurements on appropriately derivatised molecules. Towards the end of the project, we plan to perform velocity-map imaging measurements on photofragments ejected from crystalline MALDI matrices prepared at a surface, and to compare the results with data from the gas-phase studies. Improved understanding of the molecular mechanism of MALDI will help users to control and optimize the process for different analytical applications in a logical and systematic way. The project is a collaboration between the research groups of Claire Vallance, based in the Department of Chemistry at the University of Oxford, and Josephine Bunch, co-director of the National Centre of Excellence in Mass Spectrometry Imaging (NiCE-MS) based at the National Physical Laboratory. Aims and objectives: 1) VMI of isolated MALDI matrix molecules in the gas phase in order to understand their decomposition pathways following absorption of ultraviolet light. 2) Investigation into the dependence of the observed photodynamics on photolysis wavelength. 3) Investigation into the dependence of the observed photodynamics on molecular structure. 4) If time permits, investigation into the photodynamics of MALDI matrix molecules excited within a solid-state sample on a surface, and comparison with the results of the gas-phase studies. The proposed research will benefit from two patented technologies developed at Oxford and previously funded by the EPSRC. Pixel Imaging Mass Spectrometry (PImMS), uses an event-triggered, time-stamping image sensor to record the position and arrival time for each detected ion with a precision of 12.5 ns, allowing images to be recorded for ions of each mass-to-charge ratio during every time-of-flight cycle in a laser-initiated experiment. The second technology is a fast scintillator that enhances the time resolution of typical ion detector arrays. These two technologies make feasible a broad-ranging exploration into the photodynamics of a variety of MALDI matrix molecules under a number of different experimental conditions. This project falls within the EPSRC chemical reaction dynamics and mechanisms portfolio as well as the sensors and instrumentation portfolio of the EPSRC's Physical Sciences theme.